Replace \r\n by \n in Sendmail transport.

This change fixes the Sendmail transport to use \n line endings instead of SMTP-style \r\n line endings. Sendmail often does not understand SMTP line endings and requires Unix style newlines instead.

This is a replacement of the noCR option which did not turn out to work reliably. This should fix #209.

This solution uses a Transform stream, which is available in Node 0.10. A polyfill is available for earlier versions.